Sorry, that I revive this old thread, but I would like to add some information:

I have had a similar problem when compiling xulrunner and thunderbird. Both builds stopped with a segfault on my Mac Mini G4 w. 1GB RAM and 4GB swap space. - And yes, I always build w. -j1.

But I found a way to make the builds go through: I have in /etc/make.conf -ggdb3 set as default. As soon as I remove just this option, resulting in CFLAGS="-O2 -mcpu=G4 -maltivec -mabi=altivec -fno-strict-aliasing -pipe", the compilation goes through and everything runs just fine afterwards. Unfortunately debug info is then unavailable.

Yes but why crash with a segfault in that case? I would expect an out of memory error...

im not saying it is because of this but im quite sure that 1 gig ram+swap wont be enough for thunderbird.
As you say usually you should get a "killed by oom" or sth. similar in your logs in case the oom kicks in, not sure why it just segfaults ...

If my memory serves me well (and that's really not certain at all), some time back when I had a faulty memory-stick, reducing my available RAM from 4GiB to 2, I would quite easily run out of 'space' when building stuff such as xulrunner and/or firefox with '-j7', and I do think it would actually go segmentationing faults all over the place.

OK that's a bit of a dramatisation, perhaps not all over the place.
I should probably mention that when it did happen, the whole system (or rather x-server and/or KDE) would become quite unresponsive, freezing and stuttering, until it would 'crash'.

I do think it was around the Firefox 3 era, where I would use 1.9~ xulrunner for that.
So yeah, I concur with looking into expanding the space for the build even though I can not say for sure either, that it will help.

At least for the more recent versions of Firefox, and probably Thunderbird too, there is actually a check in place that will take a look on over the available disk-space and possible RAM too, I forget, and will warn that the build will probably fail if not enough space is found.

That's not about xulrunner, though, which isn't even required as a separate package for them any longer in the more recent builds.

Just some thoughts~_________________Kind Regards,
~ The Noob Unlimited ~